With the Boxcar Library complete, Alice and Colette prepare it for launch. The project’s sponsor, Mr. Rutherford, doubts that the loggers will read, prompting Colette to bet him that more than 20 books will be checked out in the first month. Colette, who is intensely private and ducks away when the newspaper photographer arrives, forcefully urges Alice to join the inaugural journey, accusing her of being scared. Alice declines but is unsettled by the charge.

Later, at a fundraising gala, Alice’s father carelessly remarks that unlike her adventurous late mother, Alice is like him and prefers the softer things in life—not realizing how much the comment stings her, deepening her internal conflict. Sidney Walker pulls her onto the porch, where she admits she is too scared to go on the boxcar. Sidney challenges her, stating that she should either go and live her life or stay safe, but she must stop asking others to give her reasons not to go. His blunt assessment makes Alice realize she uses people to reinforce her self-imposed cage. She apologizes to him before being called back to the party.

Sidney leads the group on an unscheduled detour to the homestead of Sofia Rossi, the widow of his wartime friend, Arthur, a war photographer. While Flo and Millie grow more suspicious of Sidney’s secrecy, he uses the family’s private darkroom. When Millie goes to fetch him for dinner, she is shocked to see him developing an old photograph of a younger Alice Monroe sitting atop a boxcar. Millie concludes from the expression on Alice’s face and the intimacy of the image that the two were in love. That evening, a sad song played by Sofia’s son triggers her grief. Millie follows the distraught Sofia, who confesses that her husband, traumatized by his work as a war photographer, died by suicide. Sofia makes an impassioned plea for Millie not to romanticize their backbreaking, impoverished life in the guide, but to tell the truth about their suffering. Moved, Millie promises to listen to her real story.

Deciding she needs a plausible excuse to leave town, Alice creates a cover story about helping an injured librarian colleague. After her father is asleep, she leaves a note in his office and successfully sneaks out of the house. Feeling exhilarated and free, she runs to the train station and boards the Boxcar Library. There, she finds Colette, who is states that she knew Alice would be unable to abandon the project she started. When Alice worries about the lack of sleeping space, Colette quickly arranges for a second mattress to be brought aboard.

As the train pulls away from Missoula, Alice commits to staying for one month, until the line reaches its end and returns. Colette, in turn, promises not to abandon the project. Alice watches her hometown disappear from the window, reflecting on the sheltered life she has left behind. She resolves to use this opportunity to build a real friendship with Colette, feeling that for the first time, she is headed in the right direction.

Millie, Sidney, Flo, and Oscar travel into Glacier National Park. During the drive, Sidney apologizes for his initial hostility toward Millie, and she suggests he apply for a photography job with the Resettlement Administration. After taking in the stunning scenery of Going-to-the-Sun Road, they meet with a naturalist named Theodore Cooke, who provides information for their guidebook and shares the story of how he came to settle in Montana: Originally from California, he found that the beauty and remoteness of the Glacier area helped him recover from alcohol dependency. Later, on a boat ride to their chalet, Millie confides in Oscar about her suspicion that Sidney sabotaged the previous iteration of the guide. Oscar agrees, revealing that Sidney took a mysterious “fishing trip” immediately after the incident. Millie then tells Oscar that Sidney recently developed a 10-year-old photograph of Alice Monroe. Oscar’s reaction suggests that this revelation confirms Sidney’s motive, though he doesn’t explain.

Alice and Colette arrive at the first lumberjack camp with the boxcar library. Alice grows anxious when no one visits, but they eventually get their first patron: A young, aspiring actor named Jonah who borrows a copy of George Bernard Shaw’s Pygmalion. Still, with only two visitors by the end of the day, Alice fears the project is a failure. To attract attention, Colette arranges an impromptu public performance of the play. That evening in the camp’s main dining hall, she, Alice, and Jonah perform a dramatic reading for the lumberjacks, with Jonah playing the female lead, Eliza Doolittle. The show is a chaotic but massive success, delighting the men. Ecstatic and filled with new hope, Alice later thanks Colette for saving the library, calling her the perfect person for the job.

In 1923, two years after her father’s death, Colette relentlessly hunts his killer. His distinctive birthmark means that many people remember him, and she travels through the state asking people about him. She has learned that he goes by several names but is most often called Petey. To survive the dangerous, solitary life of riding the rails, she travels disguised as a man, fueled by grief that has hardened into anger. She reflects on her isolation, realizing she has no home or community. In a boxcar, she meets a young man named Bobby Bryant, and they connect over his copy of Willa Cather’s O Pioneers!. Colette suggests that books create a shared community for isolated people like them. In a rare act of trust, they exchange their real names, and Bobby gives her the novel. After they part, Colette briefly considers giving up her quest and instead finding a community to settle in, but she quickly dismisses the idea, continuing her hunt.

The Boxcar Library proves to be a great success at its first stop. Alice feels fulfilled as she helps numerous lumberjacks, including a shy woman named Claire who she suspects is abused and to whom she gives a copy of Henrik Ibsen’s A Doll’s House. After Alice stands up to the superintendent’s wife, who calls the library a “den of depravity” (263), she and Colette share a meal on the boxcar roof, where Alice reflects on her own learned helplessness. While en route to the next camp, their car is boarded by an armed man who holds Alice at gunpoint and claims he is not after money. Colette aims a shotgun at him and forces him to jump from the moving train. When Alice marvels at her bravery, Colette explains that she fears powerful men in suits more than common bandits. Alice asks what the man might have wanted if not money, and Colette’s implausible theory—that he was after the books—leads Alice to suspect that Colette is keeping secrets. Two weeks pass, and Alice grows to love her new, rugged life. As the train pulls into a mining camp, she is stunned to see Sidney Walker waiting for them.

While on the porch of the Going-to-the-Sun Chalets, Oscar reveals his past to Millie. He discloses that his birth name is Nathaniel Davey and that he grew up in a mining town where Alice Monroe, the founder of the Boxcar Library, brought books out to the camps. This gift inspired him to become a writer, and he adopted the pen name Oscar Dalton to get published at a young age. He explains his deep respect for Alice’s Boxcar Library project, which she started shortly after his family moved away.

Returning to their investigation, Millie and Oscar theorize that a significant falling-out occurred between Alice and Sidney about ten years ago, possibly during a boxcar library trip to one of the towns Sidney now wants to avoid. Millie suggests finding the original boxcar librarian to learn more, but Oscar reveals that the librarian, Colette Durand, died over a decade ago.

In Miles City, Montana in 1923, two years after her father’s murder, Colette unexpectedly finds Finn Benson at a union meeting. After he speaks, they reconnect, and spend an intimate night together in his hotel room, where she tells him that she has identified one of the killers. Colette explains that she has been tracking him but is always one step behind. Impressed with her progress, Finn agrees they should share information going forward. He must leave for Chicago the next morning. Before he departs, he kisses her, but Colette pulls back, unwilling to get attached and lose focus on her mission. Sensing her emotional distance, Finn leaves with a parting comment, doubting she will ever find what she is looking for.